As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and high discharge voltage, on which much research has been carried out and which is now commercially available and widely used.
Depending upon the kinds of external devices in which the secondary batteries are used, the secondary batteries may be used in the form of a single battery or in the form of a battery pack having a plurality of unit cells electrically connected with each other. For example, small-sized devices, such as mobile phones, can be operated for a pre-determined period of time with the output and the capacity of one battery. On the other hand, a battery pack needs to be used in medium- or large-sized devices, such as laptop computers, power tools, and hybrid electric vehicles, because high output and large capacity are necessary for the medium- or large-sized devices.
The battery pack is a battery structure in which a plurality of unit cells are electrically connected in series and/or in parallel with each other. It is very important for the battery pack to maintain the stable arrangement structure of the unit cells such that the unit cells can be electrically connected with each other in sequence.
Especially, cylindrical secondary batteries, which are used as the unit cells of the battery pack, have great difficulty in maintaining the arrangement structure due to the appearance characteristics thereof. For this reason, the use of additional fixing members is necessary for the cylindrical secondary batteries.
Generally, a battery pack including such cylindrical secondary batteries is constructed such that spacers having a structure corresponding to the external shape of the cylindrical secondary batteries are disposed between the cylindrical secondary batteries so as to maintain the arrangement structure of the cylindrical secondary batteries. However, the spacers interfere with the flow of heat generated from the cylindrical secondary batteries during the charge and discharge of the cylindrical secondary batteries, thereby causing the degradation of the battery pack. Also, the spacers are mounted to the battery pack as separate members with the result that the assembly process is complicated, and the manufacturing costs are increased. In addition, the battery pack is constructed by mounting the cylindrical secondary batteries fixed to the spacers in an additional external case. However, the use of the spacers and the external case increases the volume and weight of the battery pack.
Especially, a battery pack used as a power source for portable power tools, such as electric drills, or hybrid electric vehicles greatly needs a unit for stably maintaining the arrangement structure of unit cells, even when external physical impacts, such as vibration or dropping, are applied to the battery pack, and smoothly moving a large amount of heat instantaneously generated from the battery pack due to a high rate of discharge from the battery pack.